A widespread problem of ceramic materials that are used as NTC thermistors is that the resistances of the ceramic materials have a B constant which, although it meets the requirements for thermistor sensitivity, assumes a value in the high temperature range that leads to resistance values too low in practice to be measured by measuring instruments.
An NTC thermistor is understood as meaning a current-conducting material that conducts the current better at higher temperatures than at lower temperatures. The resistance of the material decreases with increasing temperatures. These materials therefore have a negative temperature coefficient, which is why they are referred to as NTC materials. For an NTC material, or an electroceramic component that comprises such a material, a characteristic curve can be determined. This can be described by the Arrhenius relationship, which is as follows:RT=RNexp[B*(1/T−1/TN)] or ρT=ρNexp[B*(1/T−1/TN)]wherein RT stands for the resistance and ρT stands for the resistivity at the temperature T. RN stands for the resistance at an agreed nominal temperature TN, for example TN=25° C. The B constant corresponds to the quotient EA/k, wherein EA is the thermal activation energy and k is the Boltzmann constant. RT is linked to ρT by the geometry factor according to RT=ρT*Lk/Ak, wherein Ak stands for the contacted area of a plane-parallel test piece and Lk stands for the distance between the contacted areas. The thermal activation energy indicates the activation energy for the conduction of polarons and is generally related to a specific temperature interval, for example for B25/100° C. to the interval from 25 to 100° C.
Polarons are understood as the coupling of electronic charge carriers with lattice vibrations, the phonons, the linkage of which is referred to in theory as quasiparticles.
The B constant is also a measure of the temperature-dependent sensitivity α, which indicates the change in the resistance of an electroceramic component in dependence on the temperature:α=1/ρT(dρT/dT)=−B/T2 